Yeah, it isn't a prototype; it is the MSD incarnation of the mass-production model of the GM Cannon (Space Assault Type) from MSV-R! Note the extra calf thrusters for the purpose (if it was a prototype or ground-use version, it would have the heavier leg armor instead).

I think that might be Bandai's plan all along. The GM Cannon actually does exist in the Origin manga- but has a pretty different design from this (basically being an Origin GM with shoulder cannons)

Come to think about it, the Origin GM is pretty different too- its head design is very different from the "classic" GM with a goatee and unique flairs, a different shield, and using the same unique beam rifle from the Origin Gundam.